A middle Miocene relative paleointensity record from the Equatorial Pacific

Abstract

We present a high-resolution magnetostratigraphy and relative paleointensity (RPI) record derived from the upper 85m of IODP Site U1336, an Equatorial Pacific early to middle Miocene succession recovered during Expedition 320/321. The magnetostratigraphy is well resolved with reversals typically located to within a few centimeters resulting in a well-constrained age model. The lowest normal polarity interval, from 85 to 74.87m, is interpreted as the later part of Chron C6n (18.614–19.599Ma). Thirty-three other magnetozones occur from 74.87 to 0.85m, which are interpreted to represent the continuous sequence of chrons onset of Chron C5Er (18.748 Ma) to the end of Chron C5An.1n (12.014 Ma). We identify three putative previously-unrecognized subchrons within Chron C5Cn.1n, Chron 5Bn.1r, and C5ABn. Sedimentation rates vary from about 7 to 15m/Myr with a mean of about 10m/Myr. We observe rapid, apparent changes in the sedimentation rate at geomagnetic reversals between ∼16 and 19Ma that indicate a calibration error in geomagnetic polarity timescale (ATNTS2004). The remanence is carried mainly by non-interacting particles of fine-grained magnetite, which have FORC distributions characteristic of biogenic magnetite. Given the relative homogeneity of the remanence carriers throughout the 85-m-thick succession and the fidelity with which the remanence is recorded, we have constructed a relative paleointensity (RPI) record that provides new insights into middle Miocene geomagnetic field behavior. The RPI record indicates a gradual decline in field strength between 18.5Ma and 14.5Ma, and indicates no discernible link between RPI and either chron duration or polarity state.